This invention relates to infrared (IR) charge-injection devices and especially to two-dimensional, infrared, charge-injection-device arrays used in a series-parallel scan mode.
The development of charge-coupled device (CCD) and charge-injection device (CID) area-imaging arrays for .lambda. &lt;1 .mu. m has encouraged the infrared community to examine the application of these technologies to infrared focal planes. For terrestial applications the interest is primarily in the development of new kinds of thermal imagers for the 3-5 .mu.m and 8-12 .mu.m atmospheric windows. It is expected that the use of the CCD's at or near focal plane will permit the use of thousands of IR detectors in a focal plane at reasonable cost with good signal-to-noise ratios.
CCD and CID structures can be fabricated in materials sensitive to infrared radiation; however CCD and CID structures which are appropriate for the visible and very near infrared are not necessarily appropriate for thermal imaging systems. Because of the low contrast thermal imagery, a DC-coupled starting sensor would have difficulty distinguishing between variations in detector responsivity and variations in image irradiance. For example, net detector-to-detector responsivity uniformity in an unscanned array would need to be better than 0.35% in the 3.4-5.1 .mu.m band in order not to compete with apparent target-temperature differences of 0.1.degree. C.
The use of two-dimensional arrays in a mechanically scanned series-parallel configuration using time delay and integration (TDI) and an effective AC coupling of the detector with the image avoids these stringent uniformity requirements. Also TDI provides built-in redundancy in that a few dead detectors can be tolerated without significantly degrading the imagery.
The technical approach to a high-packing-density array for series-parallel scan, which do date has received the most investigation, mates conventional InSb photodiodes with a SI CCD chip to form a InSb photodiode-Si CCD sandwich focal-plane module. Such a module is shown and described in U.S. Pat. No. 3,883,437, issued May 13, 1975. Several modules would be used to form a focal plane of thousands of detectors. With this approach each detector is individually connected to a direct-injection input circuit which inputs photocurrent into a silicon TDI CCD register. This method of inputting photocurrent into a Si CCD requires a connection for each detector and has certain signal-to-noise limitations. Preamplifiers between the detector and the CCD have not been used to data due to packing density considerations.